The epidermis and stratum corneum will be studied by means of the freeze-fracture technique. The nature of the thickened horny cell membrane complex (marginal band), the lamellar structure of the membrane coating granules and the intercellular junctions will be given particular attention. Studies will be made of the permeability of normal and essential fatty acid deficient mouse skin to ultrastructural tracer molecules such as lanthanum, peroxidase and ferritin.